Chromium plating bath



7 United States Patent CHROMIUM PLATING BATH Melvin R. Zell, Cleveland, Ohio No Drawing. Application March 23, 1956, Serial No. 573,332

3 Claims. (Cl. 204-51) The invention relates in general to the electrodeposition of metallic chromium from an aqueous electrolytic bath which includes urea and more particularly to the addition of formamide to the urea type bath.

The disadvantages encountered in utilizing the so-called chromic acid process industrially for chromium plating are quite well known and many attempts have been made to produce a plating bath without the use of chromic acid. The use of chromic acid is dangerous in that it is highly corrosive in and of itself, during plating operations there is an abundant evolution of poisonous gases, and it has an exceedingly low current efficiency.

One such attempt to circumvent the use of the chromic acid process has been described and disclosed in U. S. Patent No. 2,704,273, issued to Tadashi Yoshida on March 15, 1955. In the plating bath disclosed in this patent, urea has been used as an additive in certain described proportions along with chromic sulfate and ammonium, sodium and potassium sulfate. This patent also disclosed an aging process for the bath and an anode of a particular composition. The electrodeposits produced in practicing the invention taught by the Yoshida patent are extremely brittle and are in such condition of stress that they tend to peel oif in flakes while still in the bath, especially when attempts are made to plate to appreciable thickness.

After much experimentation, it has been found that by the addition of as little as 1 ml. of formamide to a liter of any chromium salt gives a startling improvement in appearance, smoothness, covering and throwing power, and in ductility along with adherence.

It is therefore an object of this invention to provide a chromium plating bath of the urea type which is practical from the platers standpoint and which will provide a chromium plate of exceptional adherence, ductility, and appearance.

Another object of the invention is to provide a chromium plating bath wherein the use of chromic acid is obviated thereby also dispensing with the use of a highly corrosive material and also circumventing the poisonous gases resulting during the plating operation. The resulting savings in cost of original installations such as blowers, fans, etc. and containers for the chromic acid will be apparent.

Public sentiment against the use of the chromic acid process because of the obvious danger to the health of the workmen, has been on the increase for some time and will inevitably result in a drastic legislation to curtail the use of this material. It is for these reasons that an economical chromium plating bath, other than the chromic acid bath, be developed and introduced to the trade. The use of chromium salts would obviate the necessity of plating plants expending large sums of money in installing units to take care of used or contaminated liquids in the plating process.

Electrolytic bath The electrolytic bath used in the present invention contains in a liter: 26.6 to 133 grams of tri-valent chro- 'ice mium; to 500 grams of urea; 50 to 300 grams of a 100 ml. of formamide. 'Ihe bath is preferably prepared by introducing commercial chromic sulfate in water along with ammonium, potassium or sodium sulfate. Other materials which may also be used as conducting salts in the herein disclosed invention are salts of the following elements; namely, aluminum, lithium, magnesium, rubidiirm and cesium. The tri-valent chromium for the bath may be obtained by dissolving chromium sulfate in water, by starting with hexavalent chromium and reducing it down to the tri-valent form with sulfur dioxide or hydrogen peroxide, or by dissolving a chrome-alum solution in water. The materials are preferably heated to facilitate the dissolving of the materials in the water. The ma.- terials are then cooled down below F., and the urea and formamide are added to the mixture. The pH of the bath is preferably in the range of from 1.5 to 4.0 although the operator may wish to vary the pH outside of this range which has been given to vary the type of deposit for different plating needs. Although the components listed above within the limits given, produce good results, better results are obtained if the following limits are observed. Per liter 66.4 to 93.0 grams of tri-valent chromium; 300 to 400 grams of urea; 200 to 300 grams of a conducting salt and 5 to 20 ml. of formamide.

Equipment An acid proof, open cell was used having a diaphragm to separate anolyte from catholyte. The diaphragm may be of concrete or clay or other suitable materials well known in plating .art and was arranged centrally in the cell to form a central chamber and a surrounding chamber therearound. The catholyte or electrolytic bath hereinabove described was introduced into the central chamber and a cathode was hung therein which consisted of a conductor to be electrodeposited. An anolyte of dilute sulfuric acid was introduced into the surrounding chamber and platinum anodes were hung therein.

A diaphragm to separate anolyte from catholyte is highly desirable. Even though hexavalent chromium is formed quite slowly without a diaphragm, covering power and efliciency decline immediately when an anode is introduced directly into the plating bath. Furthermore the added complication of decomposing hexavalent chromium periodically would be some detriment to any industrial use.

It was found that much better cathode efliciency and covering power were obtained when the plating solution was enclosed within the diaphragm material which was in turn surrounded by the anolyte of dilute sulfuric acid. The total electrical resistance of such system was found to be actually less than that of the simple system of cathode and anode inserted directly into the bath. An anode of nearly any conducting material, not readily attacked, may be used in the anolyte. It should be readily understood that the use of the diaphragm is by no means mandatory in practicing the present invention but only used to illustrate a preferred working example of the invention.

Operation In operation the temperature of. the bath should be limited to a maximum temperature of 120 F. at all times. High temperatures will cause hydrolysis of formamide and urea causing black streaks to appear on the plate. The pH of the solution will rise very slowly with time and use but additions of sulfuric acid to lower the pH are not needed more often than in conventional nickel or acid copper plating baths. The exact pH range is, as mentioned hereinabove, quite flexible since the operator may wish to vary the type of deposit for ditierent plating needs; A low-pH give zero efficiency at low current density areas while a' high'pH reduces overall efliciency and often shows a blue color-'at low current density areas.

Urea helps prevent the hydrolysis of. the. formamide andalso overcomes the effects'of'smallamounts of formic acid which'may' occur; It'also prevents precipitation of salts from highly concentrated solutions at room temperaa tures. No-exact ratio'of' urea content to chromium content is to be implied; An excess may cause high viscosity and-resistivity of the bath but no other ill effects.

The. presence of ferrous sulfate. (about g./li'ter) improves brightness and'ductility. If'a buildup of iron is expected from plating directly'on' iron or steel, the initial addition of iron'may beomitted.

' A' practical example'illustratin'g the present invention" is as follows: An aqueous solution is used which contains per'literthe following constituents: 305 grams of chromic sulfate; 200 grams of ammonium sulfate; 400 grarns'of ureaand' 10 'ml'. of formamide; The chromic sulfate'and the ammoniumsulfate were dissolved in water by 'the'addition ofheat. The mixture was then allowed to cool down below at'least' 120 F. and the necessary amount of urea to'conform to the above referred to concentrations per liter was then added to the solution. Formamide was then added to provide a concentration of 10 ml; thereof tol liter ofsolution. The solution was then addedto the central chamber of the above described cell as catholyte; Dilute sulfuric acid was added to the outer'chamber as catholyte and aplatinum anode was suspended therein. A'brass strip was suspended in the catholyte' as a cathode. After 50 ampere-hours/liter had' been passed through the solution the chromium deposits produced. were still good. Appropriate additions of chromic sulfate and urea'plus a small amount of sulfuric acid tolower the pH were made. After three more 50 ampere-hours/liter periods along with the same additions, the solution still continued to produce excellent deposits.

It should be readily understood that the appended claims 'as well as' the-hereinabove given description formthe original disclosure.

Although this invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. A chromium plating. bath comprising an aqueous solution containing per liter 100 to 500 grams chromic sulfate, 100 to 500 gramsurea; to 300 grams of a substance selected from the group consisting of ammonium, potassium and sodium sulfate, and l to ml. formamide.

2.-A chromium plating bath comprising an aqueous solution containing perliter 26.6 to" 133 grams tri-valent,

chromium; 100 to- 500 grams'urea, 50 to 300 grams ofa substance selected from the group consisting of ammonium, potassium, and sodium sulfate, and 1 to 100' ml. formamide;

3. A- chromium plating bath comprising an aqueous solution containingper liter 26.6 to 133 grams tri-valent chromium; 100m 500' grams urea, 50 to 300 grams of a conducting salt, and 1 to 100 ml. formamide.

References Cited in thefile of this patent UNITED STATES PATENTS 526,114 Pl-acet et a1 Sept. 18, 1894 1,975,239 Ungelenk et a1. Oct. 2, 1934 2,704,273 Yoshida Mar. 15, 1955 OTHER REFERENCES Raub et al.: Metal Finishing, June 1940, page 315. 

1. A CHROMIUM PLATING BATH COMPRISING AN AQUEOUS SOLUTION CONTAINING PER LITER 100 TO 500 GRAMS CHROMIC SULFATE, 100 TO 500 GRAMS UREA, 50 TO 300 GRAMS OF A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF AMMONIUM POTASSIUM AND SODIUM SULFATE, AND 1 TO 100 ML. FORMAMIDE. 